Precision optical components having physical surface height aberrations smaller than one wavelength of the incident radiation are of strategic importance to the operation of many optical systems. Such components are very expensive because of the enormous investment of time and sophisticated equipment required to fabricate and figure optical surfaces. Conventional methods of fabricating and figuring optical surfaces involve grinding surfaces into optics using abrasives. Although modern optic grinders have better abrasives, tools and even equipment under computer control, the physical process is essentially the same as it has been for hundreds of years.
Ion etching, also known as ion sputtering and ion milling, has been tried as an alternative process to abrasion. Ion sputtering is a physical process in which an ion is caused to impinge upon a surface of an object with sufficient energy to cause atoms or molecules of the object to be liberated from its surface.
Sputtering has become popular in the semiconductor industry. However, sputtering has not found use in modifying optical surfaces because efforts to use sputtering for optic surfacing were severely limited by the ion current from the ion sources available at the time. One type of ion source used in such attempts is known as a Cockraft-Walton accelerator. U.S. Pat. Nos. 3,548,189 to Meinel et al and 3,699,334 to Cohen et al illustrate such ion sources in their disclosed devices. Cockraft-Walton as well as other ion accelerators used in such attempts are only capable of driving a maximum beam current of a few hundred microamperes and produce quite high ion energies, often a fraction of an MeV. Limitations result from the fundamental design of such ion sources. For example, such sources contain only a single aperture for ion extraction. The ion current extractable from a single aperture s proportional to the voltage applied to the aperture which in turn determines the ion energy. The use of a single aperture as in the prior art thus mandates that high voltage be applied to the ion extraction aperture which results in high energy ions in order to obtain an ion current on the order of a hundred microamperes. Due to such limitations ion beam etching has been essentially unworkable.
In the late 1970's the Kaufman ion source as disclosed in the publication, Technology of Ion Beam Sources Used in Sputtering, Journal of Vacuum Science and Technology, Vol. 15, pp 272-276, March/April 1978 by H. R. Kaufman et al was developed. The Kaufman ion source is capable of producing beam currents of a large fraction of an ampere, at energies within the 300-1500 eV range. The beam is sufficiently controllable, stable and repeatable, to be satisfactory for use in surface modification devices. A Kaufman ion source having a grid structure in accordance with the invention can produce minimum current levels of at least about 200 times and optimally about 800 to 1500 times the current level of the Cockraft-Walton and other devices used previously in ion etching. Such Kaufman ion source beam current is on the order of 30 to 400 mA versus a Cockraft-Walton device beam current of less than 0.3 mA.
The ions used in the U.S. Pat. No. 3,548,189 device are of substantially the same energy and a uniform current density is necessary. Only narrow ion beam sources are used and selective deposition in combination with selective removal is not possible. Such devices are limited to the figuring of small diameter elements because beam deflection is used as the steering mechanism, the ion source not being translatable, i.e., movable. For large diameter optics, such as those having diameters on the order of one-half meter or more, the distance from the deflection plates to the surface would have to be near the diameter of the surface. Beam current loses due to residual gas in the chamber would be great and make the process very inefficient. Too, beam dwell pattern computation is not considered in such prior art devices and methods using image processing and systems theory for optimized material removal are not applied.
In devices such as that shown in U.S. Pat. No. 3,699,334, ion beam impingement control is limited to electrostatic and magnetic deflection of the beam and to rotation of the object to be etched. In practicing the invention the ion source is itself moved. The ion sources used in the prior art are either constructed as an integral part of the vacuum system containing the object to be etched or they are external to the vacuum system and connected thereto by a tube which is evacuated with the vacuum system. No such prior art systems utilize translatable ion sources. Too, the ion beam is necessarily maintained continuously in such prior art devices in part because of the high voltages involved in extracting 20 kV to 100 kV ions. Dwell computations are based on a two step method in which the symmetrical errors need first be reduced to zero. Then isolated symmetrical errors are removed. In practicing the invention all errors, symmetrical and nonsymmetrical, are removed in one step. Nonsymmetrical and arbitrarily shaped beam objects can not be figured with such prior art devices. In addition the beam energies of the prior art devices, 20 to 100 kV, are known to damage many materials. The apparatus of the invention operates at a maximum energy of about two kV. The prior art beam taught by the U.S. Pat. No. 3,699,334 only focuses the ion beam to a diameter between one and five millimeters whereas that of the invention focuses the ion beam within a two to five centimeter and larger range to enable the correction of a wide range of sizes of surface aberrations for more efficiently than with prior art devices. The ion source used in accordance with the invention provides electrons to avoid the electric charge effects requiring a separate source of electrons in prior art devices.
Thus, it can be seen that the prior art devices and methods can not figure large surfaces and can not use both removal and deposition to figure a surface. Such devices are limited to low current, high energy, narrow beam ion sources and there is no control of beam current spatial distribution. Large and non-symmetric surfaces can not be etched by such devices and methods.